1. Field of the Invention
The present invention relates, in general, to a fuel cell system and, more particularly, to a device and method for controlling fuel cell stack connection in which a plurality of fuel cell stacks are included.
2. Description of the Related Art
In a solid oxide fuel cell (SOFC) system used for generating medium and large capacity power of several ten kilowatts (KW) to several megawatts (MW), a plurality of small capacity fuel cell stacks are connected to each other so as to form the desired medium and large capacity. The fuel cell stack has a structure in which several ten to several hundred unit fuel cells are stacked and connected together so as to create desired output power.
When using such a fuel cell stack, the following problems have arisen in recent years.
When forming a fuel cell system by connecting fuel cell stacks to each other in series, the fuel cell system is problematic in that, when a problem occurs in even one stack, the fuel cell system will be shut down. Further, when fuel cell stacks are connected together in parallel to form a fuel cell system, the fuel cell system is problematic in that, when a part of the stacks is deteriorated, the deterioration of the stack may be propagated to the other normal stacks. To avoid the above-mentioned problems, it is typical to use a fuel cell system, in which fuel cell stacks are appropriately connected together in series and in parallel.
However, the fuel cell system, in which the fuel cell stacks are connected together in series and in parallel, may be shut down as time goes by.
Another problem of the above-mentioned fuel cell system resides in that the fuel cell system is configured such that a single DC-DC converter of a power conditioning system (PCS) should treat a plurality of input voltages.
In the related art, the power conditioning system (PCS) typically includes a single DC-DC converter which functions to treat input voltages (for example, 40V to 80V) applied thereto from a plurality of fuel cell stacks. However, as the number of input voltages applied to the DC-DC converter is increased, the input voltage treatment cost of the DC-DC converter is increased, and the operational efficiency of the DC-DC converter is reduced, for example, the pulse rates (ripple rates) of the DC voltages are reduced, thereby reducing the operational efficiency of the power conditioning system.
FIG. 1 is a block diagram illustrating the construction of a related art fuel cell system, in which voltages of a plurality of fuel cell stacks connected together in parallel are input to a single converter of a power conditioning system.
As shown in FIG. 1, when assuming that voltages of a plurality of fuel cell stacks 210, 220, 230 and 240 having had their original voltages of 60V remain at 60V, or reduced to 59V, 56V and 55V, respectively, due to external or internal shock, deterioration of deteriorated fuel cell stacks, for example, the fuel cell stacks 230 and 240 that have relatively highly reduced voltages of 56V and 55V will be quickly propagated to the other fuel cell stacks 210 and 220 because all the fuel cell stacks 210, 220, 230 and 240 are connected together in parallel. The quick propagation of deterioration of the deteriorated stacks to the other reduces the operational performance of the fuel cell stacks. Further, in the fuel cell system of FIG. 1, the number of input voltages required to be treated by the single DC-DC converter 510 of the power conditioning system (PCS) 800 is increased, so the operational efficiency of the DC-DC converter 510 is reduced, thereby greatly reducing both the operational efficiency of the power conditioning system and the operational performance of the fuel cell system 10. Here, the operational efficiency of the power conditioning system 800 may be expressed by multiplying the efficiency of the DC-DC converter 510 by the efficiency of a DC-AC inverter 600.
Accordingly, to solve the above-mentioned two problems, the following technologies are required.
First, a technology of placing the fuel cell stacks having the same or similar voltages in groups and of connecting the grouped fuel cell stacks together in parallel, instead of connecting the fuel cell stacks together in series-parallel, thereby reducing deterioration of the fuel cell stacks and reducing the shutdown of the fuel cell stacks is required. Second, another technology of distributing a plurality of DC voltages, which may be commonly input to a single DC-DC converter in a related art fuel cell system, to at least two DC-DC converters, thereby efficiently treating the input DC voltages and increasing the operational efficiency of a power conditioning system is required.